[build] Remove set_sources_assignment_filter

This feature was removed from upstream GN.

Bug: 61489
Change-Id: I71910cf21439f4b457049ee57626a553b4520683
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/flatbuffers/+/492298
Reviewed-by: Aaron Wood <aaronwood@google.com>
diff --git a/flatbuffer.gni b/flatbuffer.gni
index be043f1..0c5e2a7 100644
--- a/flatbuffer.gni
+++ b/flatbuffer.gni
@@ -49,12 +49,6 @@
 template("flatbuffer") {
   assert(defined(invoker.sources), "Need sources for flatbuffers_library")
 
-  # Don't apply OS-specific sources filtering to the assignments later on.
-  # Platform files should have gotten filtered out in the sources assignment
-  # when this template was invoked. If they weren't, it was on purpose and
-  # this template shouldn't re-apply the filter.
-  set_sources_assignment_filter([])
-
   action_name = "${target_name}_gen"
   config_name = "${target_name}_config"
   source_set_name = target_name